1. Field of the Invention
The present invention relates to an injection apparatus provided with a screw rotating mechanism used in an electric injection molding machine.
2. Description of the Related Art
An injection apparatus is provided with a motor for rotating a screw. As the screw is rotated, a material such as resin is kneaded and melted. The rotatory force of the motor is transmitted to the screw through a first pulley on the output shaft of the motor, a second pulley on the axis of the screw, a belt passed between and around the pulleys, etc., for example. In this case, some reduction ratio can be obtained by making the diameter of the second pulley larger than that of the first pulley.
The reduction ratio of a power transmission mechanism that uses the conventional pulleys described above depends on the difference between the respective diameters of the pulleys. Owing to spatial restrictions, therefore, the outside diameter of the second pulley on the screw side can be increased only limitedly. Thus, the reduction ratio that can be obtained by means of the pulleys is a few tenths at the most.
In order to knead a high-viscosity material, the torque of the screw must be increased. Since the reduction ratio is limited, however, the motor must be replaced with one that enjoys higher output, in some cases. However, replacing the motor is troublesome and sometimes requires change of a motor mounting section or a control device, thus entailing higher cost.
If the motor output is enhanced to increase the torque, tension that acts on the belt increases. In consequence, the belt easily wears and produces dust or requires more frequent replacement. If the tension on the belt increases, it must be controlled rather precisely. Besides, a heavier eccentric load acts on the screw. If the eccentric load grows, the screw cannot be rotated with ease and may possibly be broken.
The reduction ratio may possibly be increased by maximizing the outside diameter of the second pulley on the screw side. If this is done, however, the width and height of the frame of the injection apparatus or structural members around the pulleys increase inevitably. Accordingly, the injection apparatus is large-sized as a whole, and entails higher cost and increased weight.
The necessary speed reducing performance of the general-purpose injection apparatus with the conventional construction depends on the power transmission elements that include the pulleys, belt, etc. In consideration of restrictions on the overall size of the apparatus or the positions of the power transmission elements relative to their peripheral parts, the transmission elements are maximized in size, in many cases. Accordingly, the reduction ratio cannot be changed with ease or can be changed only within a narrow range.
If a user of the injection apparatus makes a request for higher speed reducing capability, therefore, the reducing capability can be increased only slightly. Thus, the conventional construction cannot readily comply with the user's request at a reasonable cost.